Hmm, it makes me mad when I read posts about parents butting in like this.

Rather then getting DH to say something, i think its high time YOU had the conversation with her.
Be nice but firm " MIL, thank you for helping me so much with Olive, I know you have quite a bit of experience with babies so it is reassuring to know that she is in capable hands. Can I please ask you to not feed her anything that I don't know about. She gets plenty of nutrition and is perfectly healthy. We will decide when we want to introduce different solids into her diet."

If she comes back with any excuses you can always add the threat " I have asked you nicely not to feed DD food we have not introduced her to, if you cannot respect our/my wishes, we simply will not leave her in your care."

She needs putting back in her box and as Olives mother, you need to be the one to do it obviously your Dh isn't strong enough to stand up to his mum.

Nae x